Wavelength division multiplexed (WDM) optical communication systems are known in which multiple modulated optical signals, each having a different wavelength, are transmitted on a common optical communication path. The wavelengths of such optical signals are often in a so-called “C-band”, i.e., within a range of 1530 nm to 1565 nm. The C-band corresponds to a low loss window or range of silica based optical fibers. The optical signal wavelengths often conform to a grid, in which each wavelength is spectrally spaced from one another by a uniform spectral spacing, such as 25 GHz.
WDM optical communication systems often include optical transmitters, which output the optical signals, and an optical combiner, which combines the optical signals into a WDM optical signal that is supplied to one end of an optical communication path. At a receive end, the optical signals may be demultiplexed and supplied to corresponding optical receivers.
In many WDM optical communication systems, the optical transmitters, as well as the optical combiner are provided as discrete components.
Photonic integrated circuits (PICs), however, are known in which the transmitters and the optical combiner are provided or integrated on a common substrate. The light from each transmitter, which may include a semiconductor laser (e.g., a distributed feedback or DFB laser), may be separately modulated to carry a corresponding data stream and combined by the optical combiner to provide the wavelength division multiplexed (WDM) optical signal. PIC-based optical communication systems may have improved reliability and reduced cost compared to systems including discrete transmitter and combiner components.
PICs have been deployed whereby the wavelengths of the light supplied from each laser is substantially fixed. Accordingly, specific PICs may be designed and manufactured to provide optical signals having specific wavelengths, such that, one PIC is fabricated to supply optical signals having first wavelengths for use in a particular network implementation, while a second PIC may be fabricated to supply optical signals having second wavelengths for use in a different network implementation.
In order to reduce manufacturing costs, a common PIC having tunable lasers that may be configured to output light having different wavelengths over a relative wide spectral range or band is desired. Typically, wavelength tuning may be achieved by changing the temperature of the tunable laser. In order to tune the laser over a wide range, the temperature of the laser must also be adjusted over a wide range. If the tuning temperature extends from room temperature at the low end of the range, the tuning temperature at the high end can be excessive and could exceed the junction temperature at which the lasers reliably operate. Moreover, at higher temperatures, the laser output power may drop, particularly if the laser is a DFB laser.
Accordingly, there is a need to provide a PIC having tunable lasers that can supply optical signals having wavelengths that may be tuned over a wide spectral range but with manageable changes in temperature.